With a service for transmitting voice data between mobile radio devices in a communication network, known as the “push-to-talk”-service (PTT service), a user, after pressing a special key provided for the purpose on his mobile radio device, can transmit a voice message to a recipient (or even a number of recipients simultaneously) almost in real time. The (one or more) recipients have the opportunity to send an answer in the same way, so that this PTT service is comparable with the known “walkie-talkie” service. The disadvantage of a conventional PTT service is that a user with his mobile radio device can only ever be involved in one communication session of a communication group. If he wants to be involved in a further session he must end the session which is currently active and establish a connection to another desired communication session of another communication group. This means that it is not possible to participate in a number of PTT sessions simultaneously. Managers in particular would find simultaneous or parallel presence in a number of sessions or a number of communication groups helpful.